The key to many problems like this (with some # of true and false statements) is to look for statements that are directly contradictory - like Anubis's and Osiris's in this problem. Having found a contradiction, you know that one statement must be true and the other false, no matter which is which. Then use that information together with the total number of true and false statements to help figure out the rest. In this case, you know that the remaining statement (Horus's) must be false, so he must be the guilty one.